Marine seismic surveying

ABSTRACT

A method of performing a seismic survey of a hydrocarbon reservoir in the earth formations beneath a body of water includes deploying a seismic cable from a drum carried by a remotely operated vehicle on the seabed. The cable is deployed into a lined trench, which is formed either concurrently with cable deployment or during a previous survey, to ensure good repeatability of successive surveys of the reservoir, in order to enable changes in characteristics of the reservoir, eg due to depletion, to be monitored.

This invention relates to seismic surveying, and is more particularlyconcerned with the use of seismic surveying for monitoring hydrocarbonreservoirs beneath the seabed.

In reservoir monitoring, two or more sets of seismic data signals areobtained from the subsurface area containing the reservoir by conductingtwo or more seismic surveys over the area at different times, typicallywith time lapses between the seismic surveys varying between a fewmonths and a few years. The acquisition and processing of time-lapsedthree dimensional seismic data signals over a particular subsurface area(commonly referred to in the industry as “4D” seismic data) has emergedin recent years as an important new seismic surveying methodology.

The purpose of 4D seismic surveys is to monitor changes in the seismicdata signals that can be related to detectable changes in geologicparameters. These (not necessarily independent) geologic parametersinclude fluid fill, propagation velocities, porosity, density, pressure,temperature, settlement of the overburden, etc. Of primary interest arechanges taking place in the reservoir itself. Analysing these changestogether with petroleum production data assists the interpreter inunderstanding the complex fluid mechanics of the system of migrationpaths, traps, draining or sealing faults making up the reservoir. Thisprovides information regarding how best to proceed with the exploitationof the reservoir, for example where to place new production wells toreach bypassed pay zones and where to place injectors for enhanced oilrecovery, and so helps to produce an increased quantity of hydrocarbonsfrom the reservoir, often in a more cost effective way.

An important precondition to being able to map detectable changes ofgeological parameters is that the sets of seismic data signals whichhave been acquired at different times must be calibrated so they matcheach other. The phrase “match each other” in this context means thatimages of the seismic data signals reflected from places where nogeological parameter changes have taken place must appear substantiallyidentical in the different seismic data signal sets. This requires ahigh degree of repeatability between the successive surveys.

In one prior art method of reservoir monitoring, trenches are formed inthe seabed above the reservoir, and seismic cables each having aplurality of seismic sensors distributed along its length arepermanently installed in the trenches. A seismic survey of the reservoiris then periodically performed, eg at six or twelve month intervals, byconnecting the cables to a seismic survey vessel, typically using aremotely operated vehicle (ROV) on the seabed to effect the connection,and then repeatedly operating a seismic source, eg an airgun array,which is towed by another vessel back and forth in the water above thereservoir.

This prior art method of reservoir monitoring provides quite a highdegree of repeatability, but suffers from the disadvantage that the costof the installed cables is very high, typically several tens of millionsof dollars, in relation to the amount of use obtained from them. It isan object of the present invention to alleviate this disadvantage.

According to one aspect of the present invention, there is provided amethod of performing a seismic survey of earth formations beneath a bodyof water, the method comprising the steps of:

-   -   (a) deploying a seismic cable having a plurality of seismic        sensors distributed therealong in close proximity to elongate        locating means provided at the bottom of the body of water;    -   (b) operating an acoustic source in the body of water to produce        seismic signals which enter the formations;    -   (c) detecting seismic signals which return from the formations        with said sensors; and    -   (d) removing the seismic cable from the bottom of the body of        water.

The method preferably further includes subsequently re-deploying thesame or another such seismic cable in close proximity to the locatingmeans and repeating steps (b), (c) and (d).

It will be appreciated that the method of the invention permits thedeployment of a seismic cable in substantially the same position at thebottom of the body of water for successive surveys, thus providing adegree of repeatability similar to that achieved with the abovementionedprior art method. However, between the successive surveys, the seismiccable is freed for use for other purposes, eg for monitoring otherreservoirs.

The method may also include the step of providing said locating means atthe bottom of the body of water, either prior to or substantiallyconcurrently with the first performance of step (a).

In a preferred implementation of the invention, the providing stepincludes providing locating devices at intervals along the length of thelocating means, each locating device serving to uniquely identify arespective point along the length of the locating means.

The locating means may comprise an elongate metallic member such as acable or rail, in which case the providing step comprises securing saidmember to, or at least partly burying said member in, the bottom of thebody of water.

Alternatively or additionally, the locating means may comprise a trench,in which case the providing step comprises forming said trench in thebottom of the body of water, and step (a) comprises laying the seismiccable in the trench.

Advantageously, the trench forming step includes lining the trench.

According to another aspect of the invention, there is provided a methodof performing a seismic survey of earth formations beneath a body ofwater, the method comprising the steps of:

-   -   (a) forming a trench in the bottom of the body of water;    -   (b) lining the trench;    -   (c) deploying a seismic cable having a plurality of seismic        sensors distributed therealong in the lined trench;    -   (d) operating an acoustic source in the body of water to produce        seismic signals which enter the formations; and    -   (e) detecting seismic signals which return from the formations        with said sensors.

In either of the first two aspects of the invention, the trench ispreferably lined with a relatively soft, relatively high frictionmaterial, such as nylon-reinforced rubber or plastic matting, which ispreferably brightly coloured to enhance its visibility at the bottom ofthe body of water. The material used to line the trench may be made inpre-formed sections, which are preferably installed in the trench so asto be substantially acoustically de-coupled from each other, for exampleby leaving small gaps between them.

The trench forming step may also include providing the trench with anopenable lid, which is also preferably brightly coloured to enhance itsvisibility at the bottom of the body of water.

The trench forming step may further include providing the trench withguide means, such as a wire, to guide the seismic cable into the trench.

Advantageously, the method includes blowing debris from the trenchimmediately prior to laying the seismic cable therein.

Conveniently, the step of deploying the seismic cable is carried outusing an autonomous or remotely operated vehicle at the bottom of thebody of water, while the trench (or other locating means) is preferablyprovided by the same vehicle.

According to yet another aspect of the invention, there is provided amethod of performing a seismic survey of earth formations beneath a bodyof water, the method comprising the steps of:

-   -   (a) deploying a seismic cable having a plurality of seismic        sensors distributed therealong at the bottom of the body of        water;    -   (b) operating an acoustic source in the body of water to produce        seismic signals which enter the formations; and    -   (c) detecting seismic signals which return from the formations        with said sensors;    -   wherein the step of deploying the seismic cable comprises        deploying the seismic cable from a drum of seismic cable carried        by an autonomous or remotely operated vehicle at the bottom of        the body of water.

The invention will now be described, by way of example only, withreference to the accompanying drawings, of which:

FIG. 1 is a somewhat diagrammatic illustration of a seismic survey beingcarried out by a method in accordance with the present invention;

FIG. 2 is a schematic plan view of the seismic survey illustrated inFIG. 1;

FIG. 3, made up of FIGS. 3A to 3C, and 4 are schematic representationsof alternative underwater vehicles that can be used in setting up theseismic survey illustrated in FIG. 1;

FIGS. 5 and 6 illustrate alternative forms of trench used in the methodillustrated in FIG. 1; and

FIGS. 7 and 8 are enlarged views of a liner for the trench of FIG. 5.

The survey illustrated in FIGS. 1 and 2 is designed as one of a seriesof periodic surveys for monitoring a hydrocarbon reservoir 10 beneaththe bottom 12 of a body of water 14. At the start of the series ofsurveys, several shallow parallel trenches 16 are formed in the bottom12, as will hereinafter be described. Then, for each survey, arespective seismic cable 18 is deployed in each trench for the durationof the survey, as will also hereinafter be described.

Each seismic cable 18 comprises a plurality of cylindrical metal sensorhousings 20 each containing three mutually perpendicular geophones 22and a hydrophone 24, the sensor housings being interconnected by andsubstantially uniformly spaced along an electrical cable 26. One end ofthe electrical cable 26 is connected to a buoy 28 at the surface 30 ofthe body of water 14, each buoy 28 being equipped with a radiotransmitter 32.

To conduct a survey of the reservoir 10, a seismic survey vessel 34 towsa seismic source 36, typically comprising one or more arrays of airguns, back and forth above the area containing the reservoir 10, alongsubstantially parallel shooting lines which are typically (but notnecessarily) either parallel or perpendicular to the seismic cable 18.The source 36 is operated (fired) periodically, typically every 4 to 60seconds, and the seismic signals produced travel downwardly through thewater 14 and into the reservoir 10, where they are reflected. Thereflected signals are detected by the geophones 22 and the hydrophones24 in the seismic cables 18, and the detected signals are digitised andtransmitted via the electrical cables 26 to respective ones of the buoys28. The transmitters 32 in the buoys 28 then transmit the detectedsignals to a receiver 38 on the vessel 34, for on-board processingand/or recording in known manner.

To form the trenches 16 and deploy the cable 18, a remotely operated,electrically powered, wheeled underwater vehicle, indicated at 40 inFIG. 3A and hereinafter referred to as an ROV, is lowered by crane fromthe vessel 34 to the bottom 12. The ROV 40 is connected to the vessel 34by a power and control umbilical 42, and is provided with one or moreunderwater video cameras 44 to permit an operator on the vessel tovisually control the ROV. At its front end, the ROV 40 is provided withtrench-forming means 46 for forming a trench 16 as the ROV is drivenforwardly, while at its rear end, the ROV carries a coil of a soft,relatively high friction, trench-lining material, eg of nylon-reinforcedplastic or rubber matting, and means for uncoiling the lining materialand pressing it into the just-formed trench. The ROV 40 also carries adriven cable drum 48 containing several thousand metres of the seismiccable 18, and from which the seismic cable is deployed into the linedtrench 16. The cable drum 48 is driven to try to ensure as much as ispossible that the deployed seismic cable 18 is not laid under tension inthe trench 16.

As an alternative to the wheeled ROV 40, tracked versions, indicated at40 a in FIGS. 3B and 40 b in FIG. 3C, can be used. The tracked ROV 40 bof FIG. 3C has a dome shaped to store the seismic cable 18 and a tiltedwheel to enable efficient spooling of the cable.

An alternative to using the ROVs 40 and 40 a is to use autonomousunderwater vehicles powered from and connected to transmit data torespective ones of the buoys 32, as indicated by the brackettedreference numbers in FIG. 3A.

As another alternative to the ROVs 40 and 40 a, a steerable, butunpowered, wheeled plough-like vehicle, indicated at 50 in FIG. 4, canbe used, the vehicle 50 being towed along the bottom 12 by a tow cableextending up to the vessel 34. This has the advantage of not requiringelectric power at the level required to drive the ROVs 40 and 40 a alongthe bottom 12 to be supplied via the umbilical 42.

Suitable underwater vehicles which can be used as a basis for buildingthe ROVs 40, 40 a, 40 b and 50 are available, for example, from SoilMachine Dynamics Limited, of Newcastle-upon-Tyne, England.

As shown in FIGS. 5 and 6, the trenches 16 can be V-shaped or U-shapedin cross section, the former being preferred because it ensures goodcoupling between the sensor housings 20 of the seismic cable 18 and thebottom 12 for a wide range of V angles and trench depths.

The trench-lining material, indicated at 52 and 54 in FIGS. 5 and 6respectively, is brightly coloured to enhance its visibility when viewedvia the video camera or cameras carried by the ROV, and has passivetransponders (not shown) embedded in it at intervals of, for example,12.5 metres. These transponders can be of the type described in our PCTPatent Application No WO 00/03268, and each of them contains a uniquecode defining the position of the transponder along the length of therespective trench 16. A non-contact inductive reader is carried by theROV to energise and read the transponders, as will become apparent.

FIGS. 7 and 8 show the lining material 52 for the V-shaped trench ofFIG. 5 in more detail. As can be seen, the material is initially formedas an elongate flat rectangular sheet 56 having a steel wire 58 mouldedinto it along its longitudinally extending centre line, and two lines ofperforations 60 disposed symmetrically on each side of, and extendingparallel to, the steel wire 58. The steel wire 58 and the perforations60 define respective fold lines enabling the sheet 56 to be folded intothe V-shape of the trench 16, with two side portions or flaps 62 whichlie on the bottom 12 when the cable 18 is being deployed into thetrench. These flaps 62 can also be folded over the top of the trench 16,to serve as a lid to limit the ingress of sand or other debris. Thesteel wire 58 serves as additional weight to hold the lining material 52in the trench 16, and also as a means for finding the trench inconditions of poor visibility, as will become apparent.

Once the survey of the reservoir 10 has been conducted as describedearlier, the seismic cables 18 are rewound onto their respective cabledrums on the or each underwater vehicle, which is then recovered ontothe vessel 34. As a result, the very expensive seismic cables 18 areavailable for use elsewhere, eg for monitoring another reservoir in adifferent location.

As an alternative to recovering the cables 18 onto the or eachunderwater vehicle, the cables can be recovered directly onto drums onthe vessel 34.

When it is desired to survey the reservoir 10 again, the vessel 34 (orone like it) deploys an underwater vehicle similar to one of thosedescribed earlier and carrying the seismic cable 18 (or a similarseismic cable) onto the bottom 12 above the reservoir 10, as near aspossible to the start of one of the trenches 16. The brightly colouredtrench-lining material 40 facilitates visual location of the trenches 16via the vehicle's video cameras. However, if visibility is very poor,the underwater vehicle can be provided with a metal detector to detectand follow the steel wire 58. In either case, transponders can be readto determine how far the vehicle is along the trench 16 in the event itis not initially positioned adjacent one end.

Having located one end of a trench 16, the vehicle deploys the seismiccable 18 into the trench, first opening the lid (if provided) andblowing out any sand or other debris that may have accumulated in thetrench with a water jet.

Once all the seismic cables 18 have been deployed, the reservoir 10 isagain surveyed as described earlier, whereupon the cables 18 are againrecovered.

It will be appreciated that the invention has the advantage that thesecond survey is conducted with the seismic cables 18 in substantiallythe same positions they occupied during the first survey, so thatrepeatability is excellent. The same is also true for subsequentsurveys.

Another advantage is that if the first or a later survey shows that someparts of the reservoir 10 are or have become less significant thanothers, subsequent surveys can be confined to imaging the moresignificant parts, saving both time and money.

Also, later surveys can reap the benefits of improvements in seismiccable technology, in that up-dated versions of the seismic cable 18 canbe used for the later surveys.

Many modifications can be made to the described implementation of theinvention.

For example, the trench-lining material 54 can be provided with anintegral hinged lid similar to that constituted by the flaps 62 of thematerial 52, as shown at 62 a in FIG. 6. Alternatively, thetrench-lining material 52 or 54 can be made in pre-formed sections,which are deployed with small gaps between the sections to reduce thepossibility of acoustic coupling between the sections.

The purpose of the trenches 16 is two-fold: to provide good acousticcoupling between the seismic cables 18 and the bottom 12, and to ensurethat the seismic cables 18 are deployed as nearly as possible in thesame positions on the bottom 12 for each survey. In cases where thebottom 12 is relatively firm and flat, so that good coupling isrelatively easily achievable, the trenches 16 can be omitted altogether,and replaced by respective metal cables or rails which are buried in orsecured to the bottom 12 along the same lines as are shown in FIG. 2 forthe trenches. These cables or rails are deployed at the start of thefirst survey by an underwater vehicle similar to one of those describedearlier, which simultaneously lays the seismic cables 18 parallel to andin close proximity above or beside respective ones of the cables orrails.

In the cable or rail implementation of the invention, the aforementionedtransponders can be bonded to the cables or rails. Alternatively, thetransponders can bonded to pegs which are hammered into the bottom 12immediately adjacent the cables or rails, and which serve as guides forthe seismic cables 18.

For subsequent surveys, the cables or rails are detected by a suitablemetal detector carried by the ROV, and the positions along themidentified via the transponders. The ROV then uses its metal detector toguide it along the cable or rail, while simultaneously deploying theseismic cable 18 above or alongside and in close proximity to thedetected cable of rail.

In another modification, the buoys 28 are replaced by a single recordingvessel which carries a seismic data recording and processing system andwhich remains stationary during the survey. In this case, the vessel 34can be a simpler vessel, since it is required only to tow the source 36.The seismic cables 18 can be connected to the stationary recordingvessel either via respective riser cables, or by an individual risercable for, say, each pair of adjacent seismic cables 18. Or the seismiccables 18 can be connected to a common backbone cable, which can ifdesired be permanently installed on the bottom 12, with a single risercable being used to connect the backbone cable to the stationaryrecording vessel.

Finally, instead of deploying the seismic cables 18 from drums carriedon underwater vehicles, they can be deployed from drums on a surfacevessel, and guided into position on the bottom 12 by a simple ROV, or byan autonomous underwater vehicle, or by a weight, eg a powered weight,co-operating with a respective guide cable provided in each trench 16.

1-25. (canceled)
 26. A method of performing a seismic survey of earthformations beneath a body of water, the method comprising the steps of:(a) deploying a seismic cable having a plurality of seismic sensorsdistributed therealong at the bottom of the body of water; (b) operatingan acoustic source in the body of water to produce seismic signals whichenter the formations; and (c) detecting seismic signals which returnfrom the formations with said sensors; wherein the step of deploying theseismic cable comprises deploying the seismic cable from a drum ofseismic cable carried by an autonomous or remotely operated vehicle atthe bottom of the body of water.
 27. A method as claimed in claim 26,wherein the vehicle is towed by a vessel at the surface of the body ofwater.
 28. A method as claimed in claim 25, wherein the vehicle iscontrolled and/or supplied with electrical power from a vessel or buoyat the surface of the body of water.
 29. A method as claimed in claim26, further including subsequently re-deploying the same or another suchseismic cable in close proximity to a locating means and repeating steps(b), (c) and (d).
 30. A method as claimed in claim 26, further includingthe step of providing a locating means at the bottom of the body ofwater.
 31. A method as claimed in claim 30, wherein the providing stepis performed prior to or substantially concurrently with the firstperformance of step (a).
 32. A method as claimed in claim 31, whereinthe providing step includes providing locating devices at intervalsalong the length of the locating means, each locating device serving touniquely identify a respective point along the length of the locatingmeans.
 33. A method as claimed in claim 32, wherein the providing stepcomprises securing an elongate metallic member to, or at least partlyburying the member in, the bottom of the body of water.
 34. A method asclaimed in claim 30, wherein the providing step comprises forming atrench in the bottom of the body of water, and step (a) comprises layingthe seismic cable in the trench.
 35. A method as claimed in claim 34,wherein the trench forming step includes lining the trench.
 36. A methodof performing a seismic survey of earth formations beneath a body ofwater, the method comprising the steps of: (a) deploying a seismic cablehaving a plurality of seismic sensors distributed therealong at thebottom of the body of water; (b) providing a locating means at thebottom of the body of water; (c) operating an acoustic source in thebody of water to produce seismic signals which enter the formations; and(d) detecting seismic signals which return from the formations with saidsensors; wherein the step of deploying the seismic cable comprisesdeploying the seismic cable from a drum of seismic cable carried by anautonomous or remotely operated vehicle at the bottom of the body ofwater.
 37. A method as claimed in claim 36, wherein the providing stepis performed prior to or substantially concurrently with the firstperformance of step (a).
 38. A method as claimed in claim 37, whereinthe providing step includes providing locating devices at intervalsalong the length of the locating means, each locating device serving touniquely identify a respective point along the length of the locatingmeans.
 39. A method as claimed in claim 38, wherein the providing stepcomprises securing an elongate metallic member to, or at least partlyburying the member in, the bottom of the body of water.
 40. A method asclaimed in claim 36, wherein the providing step comprises forming atrench in the bottom of the body of water, and step (a) comprises layingthe seismic cable in the trench.
 41. A method as claimed in claim 40,wherein the trench forming step includes lining the trench.